1. Field of the Invention
The present invention relates to piezoresistive sensors, and more particularly to sensors that have a ceramic with a formula of [Bi2O2][Am−1BmO3m+1] (m=2, A is mono-, di-, or trivalent ions or a combination of them. B is transition elements such as Fe3+, Ti4+, Nb5+, Ta5+, W6+, Mo6+ or a combination of them) and the sensors that utilize [Bi2O2][Am−1BmO3m+1] as sensing elements and can be used to sense acceleration, shock, force, and pressure at very high temperatures.
2. Description of the Art
Piezoelectric materials including single crystals, polycrystalline ceramics, and thin films that deposited on various substrates have been used for the measurements of acceleration, pressure, and force in different application environmental conditions. When piezoelectric elements are strained by an external force or pressure, displaced electrical charge accumulates on opposing surfaces. This electrical charge can be tested and used to measure the amount of pressure, force, and acceleration that the object experiences.
Piezoelectric sensors have been developed using materials such as quartz crystal and polycrystals such as lead zirconium titanate (PZT) ceramics. However, the piezoelectric materials have not been suitable for use at high temperatures. The maximum operation temperature of the sensors made from such materials is limited by their phase transition temperature or/and Curie temperature Tc. For example, the operation temperatures of the existing sensors based on quartz are around 150° C. due to the phase change at higher temperature. PZT ceramics has Tc of ˜300° C. and the operation temperatures of the sensors are limited at ˜200° C. Bismuth titanate (Bi4T3O12) ceramics has a Tc of ˜650° C. and the sensors can work at a temperature of ˜450° C. Composition modified bismuth titanate ceramics can have a Tc of ˜800° C. and the sensors made of the material can survive at ˜500° C.
More and more gas engines and weapon platforms will operate at higher temperatures and, due to the lack of high temperature sensors, significant amount of design goes to isolating the sensors and the sources of heat. Accordingly, there is a need for piezoelectric sensors with an extended temperature range that can be used to measure acceleration, force, and pressure to improve the efficiency of the fuels, decrease system weight, and reduce cost.
If the piezoelectric elements within a sensor have low electrical resistivity, the generated charge rapidly drains away and electronic detection of the charge is therefore compromised. Hence, a high electrical resistivity is desirable especially for low frequency applications.
Polycrystalline [Bi2O2][Am−1BmO3m+1] (m=2) ceramics has very high Curie temperature (>900° C.) and very good processability. Their piezoelectric response and resistivity can be greatly improved by donor doping. Thus the need for high temperature dynamic measurement of force, pressure, and acceleration can be met by the claimed piezoelectric materials and the sensors made of the same.
In pressure and acceleration sensors, it is desired to produce a relatively large signal power from a relatively small amount of energy absorbed from the medium. The goal is to minimize the mechanical energy necessary to produce a desired output signal. In pressure sensors, energy is absorbed from the medium as pressure deflects a diaphragm. Generally, a bar deeply notched at the center and its ends is placed across a diaphragm. Gages are placed on the plane surface opposite the notched bottoms. The strain of the bending bar is concentrated at the bottom of the notches. In acceleration sensors, energy is absorbed from the acceleration field as the seismic mass deflects relative to its reference frame. For example, a structure that is used features gages that are etched free from the substrate over an elastic hinge, a so-called “freed-gage.” With the hinge carrying the transverse load and the gages much further from the neutral axis of bending than the outer surfaces of the hinge, the gages become the most highly strained material. In both the acceleration and pressure sensor, efficiency permits high sensitivity via a small physical size.
A common approach taken by manufacturers of transducers has been to create a large field of strained surface and to place onto the more strained areas strain gages of a convenient size. Alternatively, structural means have been used to concentrate strain in piezoresistors. In piezoresistive sensors, signal is produced by changing the resistance of one or more strain-sensitive resistors excited by an electric current. Hence, in a simple plane diaphragm pressure sensor with embedded gages, much of the periphery and a broad area of the center are brought to the state of strain needed to provide signal in the gages. Although gages are placed in areas of highest strain, much of the strain energy is expended in the periphery and center areas which lack strain gages.
In a freed-gage structure only the piezoresistive material sees the full level of strain; the hinge and force-gathering structures are much less strained. Though the freed strain gage was an improvement over previous strain gages, it is still not the optimal structure to detect strain. Manufacturing tolerances impose a minimum cross section on the freed-gage; hence, for the required signal power, some minimum amount of material must be strained.
Pressure sensors are used in a variety of areas, such as automotive and industrial applications, to provide an electrical signal corresponding with a measured fluid pressure. For example, pressure sensors can be used to measure automotive oil pressure and hydraulic fluid pressure.
Ceramic sensors typically have a ceramic element that undergoes a change in an electrical characteristic in response to a change in a detected parameter. Ceramic pressure-measuring cells are advantageously used in pressure measurement technology, since ceramic pressure-measuring cells have a measuring accuracy which is stable over a very long time. One reason for this is the solid ionic bonding of ceramic, which makes the material very durable and undergo virtually no ageing in comparison with other materials, for example metals. However, in comparison with metal, ceramic pressure sensors have a rougher surface and are often restrained by means of a generally nonreplaceble seal made of an organic material, for example an elastomer, in a pressure-tight manner in a housing which can then be fastened at a measuring location by means of a process connection.
There is a need for improved piezoresistive sensors that have ceramic elements.